Cyclosporin A (CsA) is an immunosuppressive agent currently being used worldwide for solid organ transplantation. It is a highly lipophilic compound which is poorly absorbed. The oral bioavailability of CsA directly influences the patient's systemic exposure to the drug. The incidence of rejection and one year allograft survival are directly related to oral bioavailability of CsA. Patients suffering rejection episodes have significantly lower maximum CsA concentrations and systemic exposure to CsA. Neoral is an improved microemulsion formulation of CsA. It has become accepted that Neoral is beneficial in the de novo patients. There is no data to show that Neoral has the same pharmacokinetic advantages in stable renal patients. This study will examine the relationship between cyclosporine variability and the clinical outcomes pertinent to transplant recipients. The primary objective of this study is to determine the intra-patient variability in cyclosporine pharmacokinetics of stable renal transplant patients treated with Sandimmune and then converted to Neoral as primary immunosuppressive therapy and to examine the relationship of variability in cyclosporine pharmacokinetics on clinical outcome, both retrospectively and prospectively, in order to examine the relationship between cyclosporine PK variability and the incidence of rejection, infection, nephrotoxicity, and hypertension. The secondary objective is to evaluate the safety and efficacy of conversion to Neoral in renal transplant patients who are between one and six years postransplant. This study has been closed by the sponsor.